1. Field of the Invention
The present invention relates to an intake- and/or exhaust-valve timing control system for internal combustion engines which is variably capable of controlling the intake- and/or exhaust-valve timing depending upon the operating state of the engine, for example the magnitude of engine load and/or engine speed.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance according to the operating state of the engine.
As is generally known, the valve timing is determined such that optimal engine performance is obtained, however the predetermined valve timing is not suitable under all operating conditions. For example, when the engine is operating within a range of low revolutions, higher torque will be obtained with an intake-valve timing earlier than the predetermined valve timing.
Such a conventional intake- and/or exhaust-valve timing control system for internal combustion engines has been disclosed in U.S. Pat. Nos. 4,231,330 and 4,535,731. In these conventional valve timing control systems, a cam sprocket is rotatably supported through a ring gear mechanism by the front end of a camshaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. The ring gear is normally biased in the axial direction of the camshaft by means of a return spring, such as a coil spring. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and the cam sprocket (and consequently, the phase angle between the camshaft and the engine crankshaft) is relatively varied. The ring gear moves as soon as one of the two opposing forces acting on it, namely the preloading pressure of the above spring means or the oil pressure applied from the oil pump to the ring gear, exceeds the other. However, in this conventional valve timing control systems, a hydraulic circuit serving as a ring gear driving hydraulic circuit functions to feed a controllable oil pressure to a pressure chamber defined at the one end of the ring gear and in addition to feed an engine lubricating oil for lubricating rotational frictional surfaces between a cylinder head, a bearing member, and a camshaft journaled by the cylinder head and the bearing member. That is, a single hydraulic circuit is commonly utilized to provide the axial sliding movement of the ring gear and to lubricate the frictional surfaces between the camshaft, the cylinder head and the bearing member.
For example, even when an oil supply to the pressure chamber is blocked during a low engine load, a small magnitude of working fluid pressure is still maintained for continuously supplying lubricating oil to the rotational friction surfaces between the camshaft and the bearing member. In the previously noted construction of the conventional valve timing control system, the slight magnitude of fluid pressure is continuously retained in the hydraulic circuit, with the result that working fluid in the pressure chamber is not exhausted smoothly and quickly and therefore quick, axial sliding movement of the ring gear is prevented when an oil supply to the pressure chamber is stopped. Consequently, the prior art valve timing control system exhibits a low step-response characteristic with regard to an intake- and/or exhaust-valve timing control executed by a variable valve timing control system.